Abstract: Using a variety of testing methods, the microstructure of the expansive soils dehumidified to different water contents is studied. The evolution characteristics of the multi-level microstructure of the expansive soils are discussed. The test results show that: (1) The pore structure of the expansive soils presents a bimodal distribution. As the water content decreases, the pore volume gradually decreases. At high water contents levels, the soils contain more micro-cracks and overhead pores, forming a loose structure, and as the water content decreases, the flaky particle units form a stack in a surface-surface contact, and the dominant pore structure changes from inter-aggregate pores to intra-aggregate ones. (2) At the initial stage of dehumidification, the dehumidification cracks appear first on the surface of the samples. As the water content decreases, the original cracks inside the samples begin to develop dehumidified ones, and gradually expand outward, and finally penetrate each other. The connective crack rate is proportional to the crack rate. (3) With the decrease of the water content, the pores and cracks of the expansive soils develop synchronously. The decrease of pore volume mainly occurs in the early stage of dehumidification, however, the development of cracks is more obvious in the later stage of dehumidification.